1. Field of the Invention
The present invention relates to an apparatus and method to detect a blood flow signal free from a motion artifact as well as a stress test apparatus using the same, in which the motion artifact caused by the motion of an examinee is removed from the blood flow signal detected by photo-plethysmography using a predetermined base pattern, thus performing exact detection of the blood flow signal as well as exact stress measurement using heartbeat information calculated through the detection.
2. Description of the Related Art
Generally, photo-plethysmography (hereinafter, referred to as “PPG”) is used to detect a blood flow signal associated with a heartbeat by use of a predetermined number of LEDs (Light Emitting Diodes) and photo-detectors, for example, to extract blood flow information through a contact point at which a simple sensor module is in contact with a part of a human body (e.g., a finger or an ear).
The PPG has been widely used in test equipment to check a state of an examinee with or without a medical purpose, because the examinee may be easily examined compared to a heartbeat-detection test using an electrocardiogram (ECG), where two or more electrodes must be attached to the examinee.
Specifically, a representative example of the PPG is a stress test, in which a heart rate (referred to as HR hereinafter) and a heart rate variability (referred to as HRV hereinafter) are calculated using the blood flow signal obtained through the PPG, and then a stress level is measured by information on mental and/or physical states of the examinee, which is obtained through analysis of the HR and HRV.
Meanwhile, the PPG has a drawback in that even slight motion of the examinee brings about a noticeable motion artifact. The motion artifact is a noise signal caused by shaking or vibration of the examinee during detecting the blood flow signal, acting as a serious obstacle to exact detection of the HR and HRV.
There has been a proposal to overcome the drawback in U.S. Pat. No. 5,662,106, titled OXIMETER WITH MOTION DETECTION FOR ALARM MODIFICATION, in which a predetermined threshold value, which is obtained through a waveform of a derivative signal resulted from motion of an examinee, is set, and when a motion artifact is checked, an alarm is given to the examinee.
Specifically, as shown in FIG. 1, a ratio of the height of the positive peak of the derivative signal, A, and the height of the negative peak of the derivative signal, B, is greater than 1-1.4 for the blood flow pulse signal. Accordingly, a threshold value is set to 1-1.4, and signals having values less than the threshold value are determined to be the motion artifacts.
This method has a problem in that, during operation of a timer for a predetermined time and detecting a blood flow signal through a part of the body of the examinee, if it is determined that signals caused by the motion artifact are present, the timer is reset, and then the blood flow signal must be detected again. Thus, the whole measurement time is increased, causing a burden with respect to the detection of the blood flow signal.
Further, emission of an alarm sound indicating the presence of the motion artifact may cause the examinee to have a hesitating or shaking motion. Hence, the method does not function as an effective one for exact detection of the blood flow signal from which the motion artifact is removed.
For this reason, there is a need for an effective method to detect blood flow which is capable of providing exact information on the heartbeat and stress by more rapid and exact detection of the blood flow signal without imposing a burden on the examinee.